FeRAM (Ferroelectric Random Access Memory), MRAM (Magnetic Random Access Memory), PRAM (Phase Change Random Access Memory), and ReRAM (Resistive Random Access Memory) have been developed as next-generation semiconductor memory devices.
A PRAM is disclosed in Japanese Patent Application Publication No. 2004-158854, for example. The patent publication discloses PRAM including a plurality of memory cells. Each of the memory cells is provided with a phase change element and a memory transistor.
A highly integrated PRAM called as “Chain PRAM” has been developed. The Chain PRAM includes a plurality of memory cells. Each of the memory cells is provided with a phase change element and a memory transistor. The phase change element is connected in parallel to the memory transistor. The memory cells are connected in series to one another (connected in cascade).
In fabrication of the Chain PRAM, memory transistors, phase change elements, and other parts are formed independently. This requires a larger number of manufacturing steps. A similar problem also arises in the case of ReRAM.